1. Field of the Invention
This invention relates to a light emitting device, in particular an organic electroluminescent device, and compounds primarily, although not exclusively, for use therein. These devices may be utilized in flat-panel displays.
2. Description of Related Technology
Organic electroluminescent (EL) displays (or organic light emitting diodes (OLEDs)) have been attracting much attention as a potential alternative to liquid-crystal displays for a number of flat panel display applications. In essence an organic EL device comprises a thin organic layer sandwiched between two electrodes, such that when a current is passed between the electrodes light is emitted from the organic material. The organic material can be macromolecular or a molecular species. In the most efficient devices there are typically a number of organic layers between the electrodes, for example a hole transporting layer, a luminescent layer and an electron transporting layer.
Volatile molecular compounds have the advantage that they can be deposited by thermal evaporation. This allows known techniques (e.g. shadow masking) to be used to form pixellated displays. The luminescent layer may be a homogeneous film, or may consist of a host and a dopant.
Although OLEDs have been under development since the late 1980s there is still scope for improved emissive and charge transporting materials. To make a full color display there is a need for efficient blue, red and green emitters that meet the NIST or PAL color coordinate standards, and that these materials have a long operating lifetime. The lifetime of current blue emitters is less than is required for many applications.
It is known that recrystallization of the organic films is one failure mechanism and hence that a high glass transition temperature (Tg) is a necessary requirement for a long device lifetime.
Among the best-known blue emitting compounds for organic EL devices are stilbene and triarylethylene derivatives such as 4,4′-bis(2,2′-diphenylvinyl) biphenyl (DPVBi) (Idemitsu Kosan U.S. Pat. No. 5,130,603 and App. Phys. Lett. 1995, 67(26), 3853-3855). DPVBi devices can be made more efficient by doping the DPVBi layer with a blue dye, but this adds complexity to the device fabrication. The resulting color will depend on the blue dye used. The spiro compound spiro-DPVBi has an improved Tg relative to DPVBi; however the color is still not as blue as is desired for a full-color display. The emission maximum is at 470 nm and the FWHM (Full Half Width Maximum) is greater than 80 nm (Proceedings of SPIE Vol 4105 (2001) p125-133).
It would be advantageous to enhance the color purity, efficiency and lifetime, particularly of blue emitting EL devices.
The non-planar structure of the compounds of this invention makes the compounds surprisingly volatile for their size which allows larger molecules to be used which have a greater chance of having a high Tg. This allows stable amorphous films to be formed by thermal evaporation. Some of the compounds are optically active, which can have the benefit that a mixture of enantiomers will tend to form a glassy film rather than a polycrystalline film. Alternatively a non-optically active compound may be easier to purify.
EL devices have been made with novel molecules that emit deep blue light.
The indenes included in the invention were designed to offer superior advantages over compounds that contain the more standard stilbene group. Many stilbene-containing compounds have been shown to have excellent emission characteristics in electroluminescent devices. There are however two significant disadvantages with the stilbene system. Firstly, although some twisting does occur around the axial bonds, the molecules are comparatively planar. This means that neighboring molecules can be packed closely together—a process that is well known to allow the introduction of intermolecular electronic orbitals, the broadening of electronic spectra, and the reduction of luminescent efficiency. Close approach of molecules can also enhance intermolecular electrochemical reactions, which are likely to be a significant cause of electroluminescent lifetime loss.
The second problem with stilbene systems is associated with the slightly twisted nature of the stilbene. Twisting oscillations (libration) along the axis means that there is a bigger distribution of molecular orbitals, which leads to a broadening of the electronic spectra of the material.
The compounds of the invention were therefore designed to address both these problems, with the aim of preparing a family of molecules that have narrower electronic spectra—particularly in the electroluminescence spectra—and that have better electrochemical stability that could provide longer EL lifetimes.
Indenes differ from the stilbenes by including the disubstituted bridging carbon atom. This achieves two purposes. Firstly, because half of the former stilbene is now enclosed in a 5-membered ring, the indene molecule is significantly more rigid which means that there are far fewer degrees of freedom for the molecule, leading to narrower electronic spectra. Secondly, because the two substituting groups overhang the faces of the indenes, the main molecular orbitals of the indene are protected from the approach of other molecules, so that intermolecular effects should be greatly reduced.
The idea of using bulky groups in inhibiting molecular packing has been known for a long time. A more relevant case is that of fluorene, where there are also overhanging groups that can protect the molecular faces from packing. However, the use of fluorene in place of biphenyl is a very specific process that cannot be used to enhance the performance of stilbene containing molecules, which are an extremely important separate class of materials for use in OLEDs.
By allowing the development of enhanced versions of stilbene derivatives, this invention opens up significant and novel possibilities for designing compounds for electroluminescent applications.
U.S. Pat. No. 5,840,217 discloses the use of spiro compounds as electroluminescence materials. The compounds are said to have a good solubility in customary organic solvents, improved film-forming properties and a significantly reduced tendency to crystallize. This leads to an increased service life of electroluminescence devices including such compounds.
U.S. Pat. No. 5,077,142 discloses electroluminescent devices comprising a wide variety of compounds which include an aromatic benzene ring, including unsubstituted indenes.
JP 3-168294 describes a compound in which fused cyclic systems are bound to a 5-membered ring having a tetrahedral carbon atom.
Spiro compounds for use in OLEDs are also disclosed in Spreitzel et al, Organic Light-Emitting Materials and Devices Vol 4105 (2001) pp 125-133.
U.S. Pat. No. 5,085,946 discloses the use of various cyclopentadiene derivatives in OLEDs. The substituents are not linked to form a ring system and the operating voltages of the devices disclosed are undesirably high.
JP 2001-307880 discloses spiro compounds for use in OLEDs. Such compounds can be quite difficult to synthesise and limit the substituents that can be used at the tetrahedral carbon position.